Gas discharge lamp generally requires an input power to provide the current essential for the illumination of the gas discharge lamp, and a ballast circuit to stabilize the current flowing through the lamp. Therefore, the efficiency of the ballast circuit dominates the power efficiency of the power supply for gas discharge lamp.
Among various gas discharge lamps, high-intensity discharge (HID) lamp is a widely-used electronic lamp. The HID lamp is featured by low impedance when the lamp is ignited. When the lamp is warmed up and operating, the lamp impedance will rise up to its normal value. In order to enhance the efficiency of the electric ballast during the operation stage of the lamp, the critical conduction mode is a widely-used control method for ballast circuit. Under the critical conduction mode, the OFF time of the main switch of the ballast circuit, i.e. the time period between the point where the main switch is turned off and the point where the main switch is turned on again, is flexible, and the switching frequency of the main switch is adaptable. Nonetheless, this control method is not appropriate as the lamp is just ignited and the lamp impedance is very low. If the critical conduction mode is applied to the lamp during the ignition stage of the lamp with very low impedance, the switching frequency of the main switch is very low and the transformer of the ballast circuit is prone to be saturated. As a result, the lamp is vulnerable to fatal damages. In order to prevent the transformer from being saturated, a bulky transformer is needed. Nonetheless, using a bulky transformer will increase the cost of the ballast circuit and deflate the efficiency of the transformer.
Hence, there is a need to develop a ballast control circuit for gas discharge lamp and a control method for ballast circuit to prevent the transformer of the ballast circuit from being saturated as the gas discharge lamp is just ignited and the impedance of the gas discharge lamp is very low, and enhance the efficiency of the ballast circuit as the gas discharge lamp is warmed up and the impedance of the gas discharge lamp rises up to a certain value such as its normal value. The invention can meet this need.